Coordinate Transformation for Separating the Classical Phase Integral and the Rotational Partition Function of a Small Liquid Cluster of Molecules

A liquid cluster of identical molecules is considered, and it is concluded that in the case of classical systems: (i) The macroscopic rotation can be defined irrespective of the molecular mobility. (ii) The canonical partition function is approximately separable into translational, rotational, and internal partition functions, and the numerical value of the rotational partition function is approximately the same as that of a rigid system with the same density and shape. (iii) The rotational symmetry number need not be considered when dealing with the usual phase integral of nonrigid systems, such as that used in this paper. (iv) These considerations lead to a value of about ${10}^9$ for the rotational partition function of a cluster of about 100 water molecules.